Pre-Chamber Spark Plug

ABSTRACT

A method and apparatus to maximize spark plug life in pre-chamber spark plugs operating with ultra-lean mixtures and/or elevated engine BMEP is presented. Electrode erosion is reduced by spreading discharge energy over a wider surface area, maintaining fuel concentration in the spark gap, controlling gas static pressure during discharge, and maintaining safe electrode temperature. Energy is spread via a swirling effect created by periphery holes in an end cap, resulting in a lower specific energy discharge at the electrodes. Divergently configured electrodes reduce the spark voltage at high operating pressures and the energy required for ignition. The flow field generated at the electrodes prevents electrical shorts due to water condensation and avoids misfire. The center electrode insulation provides an effective heat transfer path to prevent electrode overheating and pre-ignition. The volume behind the electrodes provides a volume for burnt products from previous combustion cycles and leads to more reliable ignition.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This patent application is a continuation-in-part of co-pending U.S.patent application Ser. No. 10/547,623, filed Aug. 30, 2005, whichclaims the benefit of PCT/US2005/020121 filed Jun. 7, 2005, the entireteachings and disclosure of which are incorporated herein by referencethereto.

FIELD OF THE INVENTION

This invention pertains to pre-chamber spark plugs, and moreparticularly to pre-chamber spark plugs for lean burn engines.

BACKGROUND OF THE INVENTION

Engines operating on gaseous fuels, such as natural gas, are commonlysupplied with a lean fuel mixture, which is a mixture of air and fuelcontaining a relatively high ratio of air to fuel. The lean fuel mixtureoften results in misfires, detonation, incomplete combustion and poorfuel economy. One factor that can lead to such events is the poorability of conventional spark plugs to effectively ignite a lean fuelmixture in the cylinder of the operating engine. More effectivecombustion of lean fuel mixtures can be achieved using a pre-combustionchamber.

Pre-chamber (i.e., pre-combustion chamber) spark plugs are used in thepre-combustion chamber and are typically used to enhance the leanflammability limits in lean burn engines such as natural gas lean burnengines. In known pre-chamber spark plugs such as the pre-chamber sparkplug disclosed in U.S. Pat. No. 5,554,908, the spark gap is confined ina cavity having a volume that is typically less than three percent ofthe engine cylinder displacement. The top portion of the cavity isshaped as a dome and has various tangential induction/ejection holes.During operation, as the engine piston moves upward during thecompression cycle, air/fuel is forced through the induction holes in thepre-chamber. The orientation of the holes creates a swirling motioninside the pre-chamber cavity.

The difference in density between the air and the fuel in conjunctionwith the swirl motion causes fuel stratification within the pre-chambercavity. With proper location of the spark gap, effective ignition can beachieved in a fuel rich area. The fast burning of fuel in thepre-chamber cavity can result in highly penetrating jets of flames intothe engine combustion chamber. These jets of flames provide the abilityto achieve a more rapid and repeatable flame propagation in the enginecombustion chamber at leaner air/fuel mixtures.

One problem that the prior art does not address is spark plug operationwith ultra-lean air/fuel mixtures (lambda>1.75) and high BMEP (BrakeMean Effective Pressure) (>18 bars). At such operating conditions, thespark plug life tends to be very short. As a result, commercializationof high efficiency and high power density gas engines is not practical.

What is not described in the prior art are the attributes andconfigurations required for the pre-chamber cavity, theinduction/ejection holes, the shape and location of electrodes thatminimize electrode erosion and maximize spark plug life, especially withultra-lean air/fuel mixtures and high BMEP. The prior art also does notaddress the issue of water condensation inside the spark plugpre-chamber and in between the electrodes causing short circuit and plugmisfire. Additionally, the prior art does not address the issue of plugsurfaces overheating and causing preignition.

The invention provides such attributes and configurations for enginesoperating with ultra-lean air/fuel mixtures and high BMEP. These andother advantages of the invention, as well as additional inventivefeatures, will be apparent from the description of the inventionprovided herein.

BRIEF SUMMARY OF THE INVENTION

The invention provides a method and apparatus to maximize spark pluglife in pre-chamber spark plugs operating with ultra-lean mixturesand/or at elevated engine BMEP. Electrode erosion is reduced byspreading the discharge energy per electrode unit surface area over awider area, maintaining fuel concentration in the spark gap, controllinggas static pressure at the time of electrical discharge, and maintainingelectrode temperature within its safe operating range.

The discharge energy is spread over a larger surface area by creating aswirling pattern in the air/fuel mixture. In one embodiment, theswirling pattern is achieved with periphery holes in the spark plug endcap that are drilled at an angle in the end cap. The swirling effectresults in a lower specific energy discharge at the electrodes bygenerating a flow field force acting upon the spark discharge andcausing the arc to move, thereby reducing the electrode erosion rate.

The spark plug electrodes are arranged in a variable configuration byshaping the ground electrode and/or the center electrode such that avariable size spark gap is created. The variable size spark gap resultsin a reduction of the spark voltage required for ignition at highoperating pressures, thereby reducing the energy required for ignition.The variable configuration also results in reliable ignition in enginesoperating at lean air/fuel ratios due to the minimum gap of the variablesized spark gap effectively concentrating fuel in a small gap.

The center electrode of the pre-chamber spark plug protrudes into thepre-chamber cavity. As a result, the center electrode is exposed to thecombustion of the air/fuel mixture in the pre-chamber cavity and theresulting increase in temperature. The ceramic insulation for the centerelectrode is designed to provide an effective heat transfer path toprevent overheating of the center electrode, which may causepre-ignition.

The volume behind the ground electrode provides a volume for burntproducts from previous combustion cycles and provides a more reliableignition especially with very lean air/fuel mixtures. This volume allowsthe burnt products to be pushed backwards when the air/fuel mixture foranother combustion cycle is drawn into the pre-combustion chamber. Thisvolume is sized such that effective ignition is achieved with very leanair/fuel mixtures. In one embodiment, the ratio between the volumebehind the spark gap and the spark plug pre-chamber volume is greaterthan the ratio between the engine combustion chamber volume and theengine displacement.

Other aspects, objectives and advantages of the invention will becomemore apparent from the following detailed description when taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart illustrating the overall steps taken to maximizespark plug life in pre-chamber spark plugs in accordance with theteachings of the present invention;

FIG. 2 is an isometric view of a pre-chamber spark plug in accordancewith the teachings of the present invention;

FIG. 3 is an enlarged view of the pre-chamber spark plug of FIG. 2;

FIG. 4 is a partial cross-sectional view of the pre-chamber spark plugof FIG. 2;

FIG. 5 a is a front view of an end-cap of the pre-chamber spark plug ofFIG. 2 illustrating induction holes in accordance with the teachings ofthe present invention;

FIG. 5 b is a cross-sectional view of the endcap of FIG. 5 a;

FIG. 6 a is a cross-sectional view of an embodiment of a groundelectrode having a divergent electrode configuration in accordance withthe teachings of the present invention along line 6 a, 6 b of FIG. 4;

FIG. 6 b is a cross-sectional view of an alternate embodiment of aground electrode having a divergent electrode configuration inaccordance with the teachings of the present invention along line 6 a, 6b of FIG. 4;

FIG. 7 is a cross-sectional view of the ground electrode of FIG. 6 awith the induction holes of FIGS. 5 a, 5 b superimposed on the groundelectrode;

FIG. 8 is a cross-sectional view of an alternate embodiment of a groundelectrode with the induction holes of FIGS. 5 a, 5 b superimposed on theground electrode;

FIG. 9 is a flow chart illustrating the steps of manufacturing apre-chamber spark plug from a standard spark plug using an adapter;

FIG. 10 is a cross-sectional view of a standard spark plug modified withan adapter to create a pre-chamber spark plug; and

FIG. 11 is a cross-sectional view of a pre-chamber spark plug with aslightly protruding endcap.

FIG. 12 is a cross-sectional view of a pre-chamber spark plug with aprotruding endcap that has perpendicular induction holes; and

FIG. 13 is a cross-sectional view of a swirler with angled inductionholes according to an embodiment of the invention.

While the invention will be described in connection with certainpreferred embodiments, there is no intent to limit it to thoseembodiments. On the contrary, the intent is to cover all alternatives,modifications and equivalents as included within the spirit and scope ofthe invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the invention provide a method and apparatus to maximizespark plug life in pre-chamber spark plugs operating with ultra-leanmixtures and at elevated engine BMEP.

FIG. 1 is a flow chart illustrating the overall steps taken to maximizethe operational life of a pre-chamber spark plug. Step 102 calls forproviding a path for the generation of field flow forces. In oneembodiment of the invention, a pre-combustion chamber of the pre-chamberspark plug is configured to receive an air-fuel mixture in a manner thatgenerates field flow forces within the chamber. Step 104 calls forproviding a volume for burnt products that result from the combustionprocess within the pre-combustion chamber. In an embodiment of theinvention, a ground electrode is positioned to create a volume for burntproducts within the pre-combustion chamber. Step 106 calls for the sparkplug electrodes to be arranged in a divergent configuration. To maximizethe life of the spark plug, step 108 calls for providing a flow field atthe time of ignition that is suitable to concentrate a richer air-fuelmixture in the vicinity of the spark gap. The flow field characteristicsare driven by the engine spark timing and the configuration of theinduction/ejection holes of the pre-chamber spark plug. High flow fieldson the order of 5-30 m/s and large advance spark timing on the order of20-40 crank angles before top dead center are preferable because thestatic pressure at the gap is reduced, and therefore, spark breakdownvoltage requirements are reduced.

Turning now to FIGS. 2-4, an embodiment of a pre-chamber spark plug 200that incorporates the structures associated with the steps of FIG. 1 isshown. Spark plugs are known in the art, so a detailed description ofthe conventional portions of the pre-chamber spark plug 200 need not bedescribed in detail herein. The spark plug 200 includes a cylindricalshell 202 and an insulator that is fitted into the shell 202 such that atip portion 204 projects from the shell 202. The shell 202 is typicallyformed from metallic material such as low-carbon steel. A centerelectrode 206 is disposed inside the insulator such that a portion atthe tip portion 204 projects from the insulator. The tip portion is usedto provide a heat transfer path from the center electrode 206 duringcombustion of the air/fuel mixture in the pre-chamber spark plug 200.

In a conventional spark plug, a ground electrode is used wherein one endis joined to the shell through, for example, welding and whose oppositeend is bent laterally such that a side face thereof faces a tip portionof the center electrode 206. Unlike a conventional spark plug, theground electrode 208 of the present invention is disc-like shaped and ismounted proximate the end 210 of the center electrode 206. A variablesize spark gap 212 is formed between the ground electrode 208 and thecenter electrode 206. The location of the ground electrode 208 dependsupon the engine volumetric ratio. The engine volumetric ratio is theratio of the total cylinder volume to the main combustion chambervolume. In one embodiment, the location of the ground electrode 208 inthe pre-combustion chamber is selected such that the ratio of the totalpre-combustion chamber volume to the volume behind the ground electrode208 (i.e., the volume opposite the endcap 214) is less than the enginevolumetric ratio. In equation form, this is written as:

$\frac{V_{p}}{V_{g}} < \frac{V_{t}}{V_{c}}$

where V_(p) is the total pre-combustion chamber volume (218 ₁+218 ₂),V_(g) is the residual volume (218 ₂) behind the ground electrode 208,V_(t) is the total cylinder volume (i.e., the volume displaced by thepiston), and V_(c) is the volume of the combustion chamber (i.e., thevolume of the cylinder having the air-fuel mixture). For example, assumeV_(t)/V_(c) is on the order of 10 in magnitude, then the ratio of V_(p)to V_(g) should be less than 10. The volume behind the ground electrode208 provides a volume for residual combustion products that have notexited the pre-combustion chamber (during a previous combustion cycle).The residual combustion products dilute the air-fuel mixture duringintake of the air-fuel mixture into the pre-combustion chamber.

Note that the spark plug temperature is a function of totalpre-combustion chamber volume. The plug temperature typically increases(i.e., becomes hotter) with an increase in volume due principally to thelarger mass of fuel being burned in the chamber. As the temperatureincreases, the likelihood of pre-ignition occurs. However, thecombustion performance generally improves with an increase in volumebecause there is an increase in hot gases to inject in the orifices,which results in more penetration into the main combustion chamber and abigger plume that enhances combustion. As a result, the actual ratio ofV_(p) to V_(g) used is based on engine characteristics and desiredperformance. For example, in one engine, a V_(p)/V_(g) of 5 may workbetter than a V_(p)/V_(g) of 3 while in another engine, a V_(p)/V_(g) of3 works better than a V_(p)/V_(g) of 5.

Unlike a conventional spark plug, the shell 202 extends beyond the end210 of the ground electrode 206. A threaded portion 214 is formed on theouter circumferential surface of the shell 202 and adapted to mount theplug 200 onto an engine block such that a portion of the shell 202extends into the pre-combustion chamber of the engine (not shown).

An endcap (swirler) 216 encloses the shell 202, resulting in apre-combustion chamber 218. The pre-combustion chamber 218 consists ofan ignitable volume 218 ₁ in front of the electrode 208 and a residualvolume 218 ₂ behind the electrode 208. Turning now to FIGS. 5 a and 5 b,the endcap 214 contains drilled holes 220, 222 for entrance of freshcharges of air-fuel and discharge of combustion products duringoperation. The hole area and effective flow coefficient is sized toinsure optimum “breathing” efficiency. For example, the hole area shouldbe big enough to allow filling of the pre-combustion chamber 218 duringsubsonic piston motion (e.g., piston is moving through top dead center)while small enough to provide a sonic velocity of discharging gas (i.e.,the plume). In one embodiment, the configuration of the endcap (swirler)that maximizes heat transfer and minimizes likelihood of pre-ignition is“flush” with the cylinder head. Depending on combustion chamberconfiguration and cylinder head design, a slightly protruding swirlercan be effectively constructed.

The center hole 220 is typically straight (i.e., parallel to thelongitudinal axis of the spark plug 200). The periphery holes 222 areangled (i.e., no axial axis of the holes 222 intersects with thelongitudinal axis of spark plug 200) to create a swirl pattern in thedischarging gas. The swirl of the gas/fuel mixture causes the arcgenerated during operation to move such that the energy in the arc isdissipated over a larger surface of the ground electrode 208 and centerelectrode 206, thereby lowering the temperature of the ground electrode208 and center electrode 206. The angles θ, α and distances d₁ and d₂are selected based upon the engine characteristics such as the speed ofthe piston stroke. The periphery holes 222 are sized in one embodimentto choke the flow in the periphery holes during discharge (i.e.,ignition in the main chamber) so that the main flow (i.e., discharge ofhot gases) occurs through center hole 220 while providing sufficientflow during intake of gases to the pre-combustion chamber 218 to providea swirling effect to help ignite the gases in the pre-combustionchamber. The swirling effect improves combustion stability, and withproper sizing, it does not produce excessive flow restriction. Theangled holes 222 result in the generation of flow field force actingupon the spark discharge as described below. In one embodiment, thediameter of the angled holes 222 is 0.060 inches and the diameter of thecenter hole 220 is 0.065 inches. The high flow velocity at the spark gapalso provides an additional benefit of sweeping away any water condensedduring engine shut-down.

Turning now to FIGS. 6 a, and 6 b, the shape of the electrode 208 isshaped to be divergent with respect to the center electrode 206. Thedivergence of the electrode 208 results in elongation of the arcdischarge and a variable size spark gap 212. For example, in FIG. 6 a,the ground electrode has a lobed shape such that the spark gap 212 isconcave with respect to center electrode 206. It can be seen that thespark gap 212 has a minimum gap size at location 224 and the spark gap212 diverges on each side of the minimum gap. In one embodiment, thesize of the gap ranges from a minimum gap in the order of 0.005 to 0.010inches for operation with a high BMEP and a maximum gap in the order of0.030 to 0.050 inches. Note that the minimum gap could be lower, butpresent manufacturing tolerances limit how low the gap can be withoutcostly manufacturing techniques. A gap of 0.005 is high enough wheremanufacturing tolerances are minimal. The variable size gap 212 iseffective in reducing spark voltage requirements (i.e., the drivevoltage) at high pressures. Additionally, the variable size providesmore reliable ignition during operation with lean air/fuel ratioconditions. FIG. 6 b shows an alternate implementation of a variablespark gap 212.

As previously indicated the present invention generates flow fieldforces acting upon the spark discharge and causes the arc to move anddistribute the spark energy onto a much larger surface area. This can beseen in FIG. 7, which illustrates the periphery holes 222 of end cap 216superimposed on the ground electrode 208. An illustration of flow fieldforces represented by arrows 300 acting upon the arc 302 is shown.Without the flow field forces, the arc would be concentrated at theminimum gap location 224. With the flow field forces, the arc moves andthe energy associated with the arc is distributed to a larger area asindicated by reference number 304. As shown in FIG. 7, the configurationof the induction/ejection holes 222 results in the flow field movingprimarily in the direction of arrows 300. With a differentconfiguration, the flow field can move in other directions. Theconfiguration of the induction/ejection holes 222 should be such thatthe flow field at the time of ignition concentrates a richer air/fuelmixture in the vicinity of the spark gap 212 to enhance operation. Notethat in addition to the configuration of the induction/ejection holes222, the flow field characteristics are also dictated by engine sparktiming. High flow fields in the order of 5-30 m/s and large advancespark timing in the order of 20-40 crank angles before top dead centerreduce the static pressure at the spark gap 212, which results in areduction of the drive voltage requirement of the spark voltage. FIG. 8illustrates another embodiment of a ground electrode 208 with flowfields acting upon the arc. Note that the center electrode 206 isrectangular instead of circular and the ground electrode 208 is toothshaped (i.e., is trapezoidal shaped).

In the above description, the pre-chamber spark plug was described interms of a one-piece shell construction (see FIG. 2). The shell may alsotake the form of a multi-piece shell construction. For example, astandard spark plug can be converted to a pre-chamber spark plug byadding an adapter to the existing shell of the standard spark plug tocreate the pre-chamber spark plug shell. Turning now to FIGS. 9-10, inone embodiment, a pre-chamber spark plug can be manufactured from astandard spark plug using an adapter 450. The adapter 450 is sized tofit ground electrode 208, end cap 216 and provide pre-chamber cavity 218with V_(p)/V_(g) as described above. The threads 452 on the spark plugshell 454 are removed via grinding or other operation (step 400). Theinner diameter of adapter 450 is machined such that surface 464 providesa mild shrink fit with respect to shell 454 where the threads 452 havebeen removed (step 402). In one embodiment, the shrink fit is on theorder of approximately 0.002 inches. The adapter is pre-heated andslipped onto the spark plug shell 454 (step 404). A fixture should beused to hold the spark plug shell 454 to the adapter 450 to ensure thereis adequate contact for the heat transfer path from the center electrode206 to the adapter 450 as indicated by the arrows (see FIG. 10). Theprimary heat transfer path is from the center electrode 206, throughinsulator 204, out the tapered seat 458, and into the cylinder head (notshown) via a gasket such as a copper gasket. The heat transfer pathprovides a path for the center electrode heat due to ignition of theair/fuel mixture in the pre-combustion cavity 218 and is important for along spark plug life and resistance to pre-ignition. The adapter 450 isintegrated with the spark plug shell 454 via welding (as indicated byreference 460) and the like (step 406). The welding process is typicallydone using Gas Tungsten Arc Welding (GTAW), which is frequently referredto as TIG welding, or other types of welding. Other techniques such asbrazing may be used provided the technique is capable of withstandingapproximately 2500 psi at 350 degrees Celsius.

After the adapter 450 is integrated, the adapter assembly is completed(step 408). The completion includes mounting ground electrode 208 in thepre-combustion chamber 218 and mounting endcap 216. Note that the centerelectrode end 210 may need to be machined if the end 210 is to be flushwith the ground electrode 208. In one embodiment, the ground electrode208 is held against an internal step with one or more seals or gaskets462. The ground electrode 208 may also be held in place with seals orgaskets on both sides of the ground electrode 208. Alternatively, theinner diameter of the adapter 450 may be threaded and the groundelectrode held in place with threads. While the end cap 216 is shownfitting within the adapter 450 (or the shell 202), it is noted that theend cap 216 may fit over the adapter 450 (or shell 202) or be flush withthe outer diameter of the adapter 450 (or shell 202).

As previously described, a slightly protruding swirler can beconstructed depending on combustion chamber configuration and cylinderhead design. Turning now to FIG. 11, in an alternate embodiment, aswirler 216′ that is slightly protruding from the end of the shell 202(or adapter 450) is shown. The swirler 216′ has a center hole 220′ andperiphery holes 222′ as described above with respect to swirler 216. Theswirler 216′ is attached to the shell 202 via welding, brazing, and thelike.

FIG. 12. illustrates a pre-chamber spark plug 230 according to anembodiment of the invention, having a protruding swirler 232 attached tothe shell 202. The pre-chamber spark plug 230 further includes a centerelectrode 206 having a longitudinal axis 234. As in the previousembodiment, the swirler 232 may be attached to the shell 202 viawelding, brazing, and the like. The swirler 232 includes a plurality ofholes 236 which may be drilled or formed in a sidewall 238 of theswirler 232.

FIG. 13 illustrates an embodiment of the swirler 232 having fourinduction holes 240 in the sidewall 238, and four hole axes 242, 244,246, 248. Other embodiments of the invention may have more or less thanfour induction holes. The induction holes 240 are located such that eachhole axis 242-248 is substantially perpendicular to the longitudinalaxis 234, and each hole axis 242-248 is angled such that no hole axis242-248 intersects the longitudinal axis 234. When the air-fuel mixtureis introduced into the pre-combustion chamber 218, the angled inductionholes 240 produce a swirling effect on the air-fuel mixture in thepre-combustion chamber 218. The exact location (i.e., on the sidewall238) and configuration (e.g., diameter, angle) of the induction holes240 is dependent on the desired flow field and air-fuel distributionwithin the pre-combustion chamber 218. In an embodiment of theinvention, the four hole axes 242-248 lie in a plane that issubstantially perpendicular to the longitudinal axis 234. In anotherembodiment, the hole axis 242 is substantially parallel to hole axis246, and substantially perpendicular to hole axes 244, 248.

From the foregoing, it can be seen that a method and apparatus tomaximize spark plug life in pre-chamber spark plugs operating withultra-lean mixtures and at elevated engine BMEP has been described. Thekey factors affecting electrode erosion include discharge energy perelectrode unit surface area, fuel concentration in the spark gap, gasstatic pressure at the time of electrical discharge, and electrodetemperature. The discharge energy has been spread across a largersurface area via the swirling effect created by the periphery holes inthe end cap. The swirling effect results in a lower specific energydischarge at the electrodes, which reduces the electrode erosion rate.Furthermore the high flow field obtained at the divergent electrode gapassures that any water condensation is swept away before the electricaldischarge occurs. The divergent configuration of the electrodesresulting from the shape of the ground electrode and/or the centerelectrode reduces the spark voltage at high operating pressures, therebyreducing the energy required for ignition, while providing reliableignition at lean air/fuel ratios. The design of the ceramic insulationfor the center electrode provides an effective heat transfer path toprevent overheating of the center electrode. The volume behind theground electrode provides a volume for burnt products from previouscombustion cycles and provides a more reliable ignition with very leanair/fuel mixtures.

The use of the terms “a” and “an” and “the” and similar referents in thecontext of describing the invention (especially in the context of thefollowing claims) is to be construed to cover both the singular and theplural, unless otherwise indicated herein or clearly contradicted bycontext. The terms “comprising,” “having,” “including,” and “containing”are to be construed as open-ended terms (i.e., meaning “including, butnot limited to,”) unless otherwise noted. Recitation of ranges of valuesherein are merely intended to serve as a shorthand method of referringindividually to each separate value falling within the range, unlessotherwise indicated herein, and each separate value is incorporated intothe specification as if it were individually recited herein. All methodsdescribed herein can be performed in any suitable order unless otherwiseindicated herein or otherwise clearly contradicted by context. The useof any and all examples, or exemplary language (e.g., “such as”)provided herein, is intended merely to better illuminate the inventionand does not pose a limitation on the scope of the invention unlessotherwise claimed. No language in the specification should be construedas indicating any non-claimed element as essential to the practice ofthe invention.

Preferred embodiments of this invention are described herein, includingthe best mode known to the inventors for carrying out the invention.Variations of those preferred embodiments may become apparent to thoseof ordinary skill in the art upon reading the foregoing description. Theinventors expect skilled artisans to employ such variations asappropriate, and the inventors intend for the invention to be practicedotherwise than as specifically described herein. Accordingly, thisinvention includes all modifications and equivalents of the subjectmatter recited in the claims appended hereto as permitted by applicablelaw. Moreover, any combination of the above-described elements in allpossible variations thereof is encompassed by the invention unlessotherwise indicated herein or otherwise clearly contradicted by context.

1. A method to extend the life of a pre-chamber spark plug comprisingthe steps of: providing a metal shell; arranging electrodes within themetal shell in a divergent configuration; providing a volume behind agap of the electrodes for burnt products from a previous ignition cycle;and configuring an endcap with a plurality of induction holes togenerate non-magnetic flow field forces that act upon an electrical arcwithin a pre-combustion chamber with sufficient force to cause theelectrical arc to move, thereby distributing electrical energy over anarea of the electrode surface greater than the area over which energywould be distributed in the absence of the non-magnetic flow field;wherein the endcap is mounted to the metal shell such that the endcapand metal shell substantially define the pre-combustion chamber; andwherein each of the induction holes has a hole axis that issubstantially perpendicular to a longitudinal axis of the pre-chamberspark plug.
 2. The method of claim 1, wherein the endcap includes fourinduction holes.
 3. The method of claim 2, further comprisingconfiguring the plurality of induction holes such that no hole axis ofthe four induction holes intersects the longitudinal axis of thepre-chamber spark plug.
 4. The method of claim 2, wherein the four holeaxes lie in a plane substantially perpendicular to the longitudinal axisof the pre-chamber spark plug.
 5. The method of claim 4, wherein any onehole axis is parallel to another hole axis and perpendicular to twoother hole axes.
 6. The method of claim 1, wherein the endcap furtherincludes an induction hole having a hole axis that is parallel to thelongitudinal axis of the center electrode.
 7. The method of claim 1,wherein the metal shell and endcap are sized such that the endcap can belocated within a combustion chamber of an engine when the pre-chamberspark plug is installed in the engine.
 8. The method of claim 1, whereinthe plurality of induction holes is configured to create a flow fieldvelocity of 5 to 30 meters per second in the pre-combustion chamber. 9.The method of claim 1, wherein the step of providing a volume behind agap of the electrodes further comprises providing a heat transfer pathto prevent overheating of at least one of the electrodes.
 10. The methodof claim 1, wherein the step of arranging the electrodes within themetal shell in a divergent configuration comprises arranging theelectrodes to have a minimum spark gap of 0.005 to 0.010 inches.
 11. Themethod of claim 10, wherein the step of arranging the electrodes withinthe metal shell in a divergent configuration further comprises arrangingthe electrodes to have a maximum spark gap of 0.03 to 0.05 inches. 12.The method of claim 1, wherein the step of arranging the electrodeswithin the metal shell in a divergent configuration comprises arrangingat least one lobe-shaped electrode within the metal shell in a divergentconfiguration.
 13. The method of claim 1, further comprising providingceramic insulation between a center electrode and the metal shell, andwherein the step of providing a volume behind a gap of the electrodesfor burnt products comprises configuring the ceramic insulation tothermally insulate an electrode.
 14. The method of claim 1, wherein thestep of providing a volume behind a gap of the electrodes comprisesproviding a volume according to the equation:${Volume} > \frac{V_{{pre} - {chamber}}}{V_{ratio}}$ whereV_(pre-chamber) is the total pre-combustion chamber volume and V_(ratio)is an engine volumetric ratio.